Recirculation of a controlled amount of diesel engine exhaust gas to the engine air intake is generally known to provide a beneficial reduction in diesel engine emissions of oxides of nitrogen NOx. Limits are conventionally imposed on the amount of recirculated exhaust gas EGR to avoid excessive engine intake air charge dilution that may degrade engine performance and increase particulate emission levels. An adequate amount of excess air must be maintained for a smoke-free combustion by imposing limits on the amount of fuel injection.
Both open-loop and closed-loop EGR control operate to deliver EGR to the engine air intake in an amount reflecting a compromise between the competing goals of minimizing NOx and particulate emissions in an engine that deliver a satisfying level of performance. The open-loop approaches deliver EGR according to an open-loop EGR schedule determined through a modeled or calibrated relationship between desired EGR and certain EGR parameters. Such open-loop approaches are sensitive to variations in that modeled or calibrated relationship, such as may result from sensor or actuator degradation over time.
Closed-loop approaches attempt to compensate for system disturbances by including some measure of the actual performance of the EGR control in the determination of a desired EGR amount. Since recirculated exhaust gases displace intake air that would otherwise be drawn into the cylinders of the diesel engine, a sensed engine mass airflow (MAF) is decreased with increasing levels of the exhaust gas recirculation. Thus, a closed-loop EGR control is known, which is responsive to the MAF. MAF is currently a sensed parameter on many conventional engine control systems. MAF is commonly generated through a sensor in the intake air path to the engine at a point after an air filter has filtered the intake air has been filtered by an air filter. Accordingly the MAF sensor is exposed to a minimal level of contaminants. Further, the MAF sensor is commonly spaced a considerable distance away from high temperature components, reducing potential sensor wear due to temperature.
Generally, the greater the amount of exhaust gases recirculated the lower the emission levels of oxides of nitrogen NOx. However, the air-to-fuel ratio of the mixture in the cylinders is decreased with increasing levels of exhaust gas recirculation. Therefore, in order to prevent undesirable smoke emissions, the amount of exhaust gases recirculated must be limited to levels that do not result in excessively rich air-to-fuel ratios that produce smoke emissions.
For preventing undesirable smoke emissions, the amount of fuel to be drawn into the cylinder during engine cycle must be lower than the upper maximum fuel that is limited in accordance with the actual performance of EGR control.
An EGR valve of the EGR control system is subject to the harsh environment of the EGR path, so that operation loss resulting from valve contamination and valve exposure to temperature may occur.
Accordingly, it would be desirable to take into account occurrence of operation loss of an EGR control valve that is subject to the harsh environment of the EGR path in determining the maximum fuel and desired EGR.